1. Field of the Invention
The present invention relates to a process for producing high quality motor fuels, especially a medium-pressure hydrocracking process, from low quality heavy oils.
2. Description of the Related Art
Along with the continuous development of the world economy, the demand of the market for the petrochemical products is continuously increasing. However, the resource of low sulfur crude oil in many countries is insufficient. Therefore there is a need to process a great amount of imported high sulfur crude oil. This sets a task in front of most refineries with FCC as the main equipment as to how to reform so as to meet the need of processing high-sulfur crude oil. The experience in processing high-sulfur crude oil in various countries shows that the hydrocracking process is a major means to convert high sulfur heavy oils. However, the high investment resulted from the high-pressure hydrocracking, equipment and the great demand for the hydrogen resource greatly limits the rapid development of the hydrocracking process. Therefore refiners are eager to find out a new process for solving this problem.
Hydrocracking is generally operated at a pressure level of 15.0 MPa and has many advantages such as high operation flexibility, high product quality, etc., but also has such disadvantages as a high investment in the construction and a high consumption of hydrogen. The disadvantages are more severe when there is lack of funds and of cheap hydrogen sources such as natural gas. However, because of the various advantages exhibited by the hydrocracking process in processing high sulfur crude oil, the hydrocracking process still possesses superior status and function to those non-hydrotreating processes, and therefore becomes one of the first choices made by refinery engineers in processing high sulfur crude oil. In order to overcome the shortcomings of the hydrocracking, technology, people started to explore long ago to find out whether it is possible to lower the operation pressure of the hydrocracking process and have made great advances. Medium-pressure hydrocracking or medium-pressure hydroupgrading technologies have been successfully developed (e.g. U.S. Pat. No. 4,971,680), with an operation pressure being about 8.0 MPa. The product quality is greatly affected in the medium-pressure hydrocracking due to the intrinsic shortcoming of low saturation extent of aromatics limited by the thermodynamic equilibrium. Particularly to the jet fuel, since a great amount of aromatics are transferred during the reaction to such a cut fraction and can not be saturated effectively, its quality specifications such as the smoking point, etc., can not meet the requirements. This greatly limits the scope of the lowering in the operation pressure of the medium-pressure hydrocracking process. Nowadays, the operation pressure industrially adopted in the medium-pressure hydrocracking processes is about 10.0 MPa, and most of the processes can not be directly used to produce jet fuel. Also, the quality of diesel is hard to attain the specifications of the World Standard III for diesel. The improving effect of lowering the operation pressure on the investment and operating cost is not distinct, but the product quality is lowed sharply. Therefore, there has not been a breakthrough in the development and application of the medium-pressure hydrocracking process for a long time.
U.S. Pat. No. 4,172,815 describes a process for producing jet fuel and diesel wherein the tail oil is completely recycled. Heavy cut fractions pass through a hydrocracking reactor, and the jet fuel fraction in the effluent is then fractionated out and partly recycled so that the smoking point of the jet fuel is raised. But this process has obvious disadvantages so that it is only applicable to those process flows wherein the smoking point of the jet fuel is relatively high and therefore an elevation thereof of only 2–3 mm would meet the requirement. However, the smoking point of the jet fuel produced in a medium-pressure hydrocracking process is generally lower than 20 mm, and therefore the use of this process is restricted under medium pressures. Especially, the recycle of a part of the jet fuel will certainly lower its yield and affect the capacity of the hydrocracking equipment or increase the investment in the hydrocracking equipment.
U.S. Pat. No. 5,026,472 discloses a process, wherein the jet fuel cut and the hydrogen-containing vapor in the effluent of the hydrocracking are separated by adjusting the pressure and temperature of a two stage vapor-liquid separator, and the separated jet fuel and a part of the hydrogen-containing vapor enter into a hydrogenation reactor for hydrogenation of the jet fuel, while the remaining hydrogen-containing vapor enters the hydrocracking reactor. Because of the post-processing of the jet fuel component, a qualified jet fuel product can be produced under a medium to high pressure. But the disadvantages are that the process flow is complex, the amount of high-pressure equipment is great, and the increase in the investment is more, so that the superiority of the medium-pressure can not be materialized. And since the oil vapor entering the refining reactor still contains a great amount of impurities such as H2S, NH3, H2O, etc., the hydrosaturation performance of the catalyst in the hydrogenation reactor of the jet fuel degrades, and the sorts of the applicable hydrogenation catalysts are limited too. For instance, most of the noble metal catalysts or non-noble metal saturation catalysts in reduced states are not applicable.
U.S. Pat. No. 5,447,621 (Hunter) discloses a hydrocracking process which involves an initial hydrocracking step. Hunter discloses that the desired fuel cuts from the hydrocracking step can be further saturated, at a temperature of from 250 to 350° C., a pressure of from about 3 to about 7 MPa, in the presence of a CoMo or NiMo base metal or a noble metal catalyst (see e.g. col. 8, lines 15–20). The elevated pressure employed by Hunter may be consistent with the knowledge of a skilled artisan in the art that the conversion of the hydrogenation reaction of aromatics increases with the increase of the reaction pressure. Nevertheless, due to the employment of the elevated pressure, Hunter requires the use of additional equipment to pressurize the industrial hydrogen source. Specifically, industrial hydrogen is generally supplied at pressure below 3 MPa (e.g. the pressure of the industrial hydrogen produced from light hydrocarbon after purification is generally about 1–2.5 MPa, and the pressure of the by-product hydrogen from the catalytic reforming of naphtha is generally about 0.8–2.5 MPa). In addition, due to the elevated hydrosaturation temperature employed by Hunter, the feed may need to conduct heat-exchange with the effluents from the hydrosaturation reactor and hydrocracking reactor to attain the temperature necessary for the reaction, which apparently cannot meet the desirability of simplifying the process flow and equipments.